LI-COR LI-8100A - Installing and Using Soil Collars; Insertion Depth

Initial Setup
2-19
3.5"
(8.9 cm)
3.9" min I.D./4.5" max O.D.
(9.9 to 11.4 cm)
4.5" ± 0.125"
(11.43 cm)
8.4" O.D.
(21.34 cm)
24°
Figure 2-17. Dimensions for 10 cm (4") and 20.3 cm (8") soil collars.
Installing and Using Soil Collars
Soil collars should be installed several hours to one day before making a
measurement. You can test to see if the flux has stabilized by making a
measurement immediately after installing the collar, and then make subsequent
measurements over time. Note, however, that the soil surface CO
2
flux depends
on the time of day, and the diurnal cycle can be quite large.
In hard or compacted soils, it may be helpful to use a knife or sharp screwdriver
blade to create a channel of appropriate depth around the circumference of the
collar before insertion. For the larger 8" collars, you may need to lay a piece of
wood across the collar and pound with a hammer.
Insertion Depth
The optimal collar height will vary with site conditions and the length of time the
collars will be used at a given site. At a minimum, the collar should be inserted
into the soil to a depth that gives a solid foundation so the collar does not move
when moving the chamber on and off the collar. As insertion depth is increased,
lateral diffusion of CO
2
in the soil column below the chamber will be reduced.
The advantage of this is that lateral diffusion can be a source of error in the
measurement (Healy et al., 1996), but the disadvantage is that as insertion depth
increases, the possibility of root shearing increases. Collars may become loose
over time and should be moved if this occurs.
Collars should extend a minimum of 2 cm above the soil surface for the 10 and 20
cm Survey chambers, and 3 cm or more for the Long-Term chamber, depending on
the slope of the site. Collars can extend above the soil more than 2 or 3 cm, but
with greater extension there is increased shading and perturbation of air movement.
Over the long term, these perturbations could result in changes of evaporation rate,
soil temperature and soil moisture.